The present invention relates to the general field of devices for adjusting a ring for synchronizing the control of pivoting vanes of a turbomachine, so that the ring is properly centered on the axis of the machine.
In a turbomachine, it is known to use one or more stages of stator vanes to adjust the flow and direction of the gas passing through the compression section as a function of the operating speed of the turbomachine. These stator vane stages comprise a plurality of vanes (referred to as “variable-pitch” vanes) capable of pivoting about their axes that connect them to the stator, in such a manner that their pitch angle can be modified as a function of the operating speed of the turbomachine.
Known devices for controlling variable-pitch vanes conventionally comprise a control member in the form of a ring surrounding the turbomachine casing and a plurality of levers or cranks, each lever having a first end connected to the control ring via a hinge and a second end mounted on the pivot of a respective vane. Synchronized modification of the angular position of the vanes is thus implemented by turning the ring about the axis of the turbomachine.
In order to improve the efficiency of turbomachines, it is necessary to increase the precision with which the pitch angle of the stator vanes is set. To do this, it is known to ensure that the ring controlling these vanes is accurately coaxial about a circular surface of the turbomachine such as its casing shell. This coaxial configuration is generally obtained by means of pads connected to the control ring by mounting devices and surrounding the casing with very little clearance.
The devices for mounting pads on the control ring must therefore enable pad position to be adjusted radially. Thus, U.S. Pat. No. 5,387,080 discloses a device for adjusting the coaxial position of a control ring by using pads whose radial positions are adjusted by screws having two threads, with their threaded portions having slightly different helical pitch angles.
With that type of device, it has been found that during the various operating stages of the turbomachine, some of the pads are no longer in contact with the casing, thus leading to vibration, causing the threads of the two-thread screw to be worn away completely. As a result it is no longer possible to adjust the clearance between the pads and the casing.
In addition, with that device, it is not possible to remove one or more pads without it being necessary to disassemble all of the parts of the variable-pitch vane stage, and in particular the control ring. Each pad is secured to a pad carrier provided with a rod that passes right through the ring and that is screwed into the ring from beneath it. Thus, because of the presence of the casing, it is not possible to remove the pad from beneath the ring.